The ECC99 is a modern twin triode. The first 9 should indicate a B7G base according to the classical naming convention but here the valve is on a B9A base. So classically it would have been an ECC89.
The design use of the ECC99 is as driver for power valves or as an output valve in headphone amplifiers etc. The anode dissipation is 3.5 Watts and the control grids have heat sink fins on the tops of the support rods. At 800 mA the heating power is substantial. The 12BH7A also has an anode dissipation of 3.5 Watts but the anode current is 11.5 mA whereas this driver valve has a current rating of 18 mA.
Face view of one anode.
The triodes end on.
A μ-Tracer plot of the ECC99. With the anode connections reversed the curves transpose indicating that the valve is better balanced than the tester.
The thin glass tube envelope is 22 mm in diameter, and excluding the B9A base pins is 62 mm tall.